This section is intended to provide a background to the various embodiments of the invention that are described in this disclosure. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Starting from 3GPP (i.e. 3rd Generation Partnership Project) Rel-11 (i.e. Release 11), eNBs (i.e. enhanced Node Bs, or evolved Node Bs) and HeNBs (i.e. Home eNBs) may be allowed to set up and use X2 interfaces between them, under certain conditions. When deploying dense LTE (i.e. Long Term Evolution) networks or in HetNet (heterogeneous network) deployments, it may be necessary to deploy several HeNBs in close proximity of the same macro eNB. In these conditions the same eNB could have dozens of X2 interfaces to neighbor HeNBs. In theory, the number of X2 interfaces to femto neighbors could grow to be very large, possibly even larger than the number of X2 interfaces to other macro eNBs. Each X2 interface is generally carried on top of an SCTP (i.e. Stream Control Transmission Protocol) association with the other node. In order to reduce memory and computational resources requirements on eNB implementations, in some cases it might be desirable to look for solutions to reduce the number of SCTP connections that an eNB has to manage due to its X2 interfaces to its neighbors, e.g. its neighbor HeNBs.
One possible way to reduce the number of SCTP connections between an eNB and its neighbor HeNBs is to introduce a gateway node (X2-GW) between the eNB and the HeNBs, co-located with the HeNB-GW (i.e. HeNB Gateway). See, for instance, R3-120437, “X2-Proxy”, Nokia Siemens Networks (Rapporteur). The X2-GW will set up a single X2 interface with the eNB and an individual X2 interface with each HeNB connected to it. It will act as a proxy for all the X2 messages, terminating some messages and transparently forwarding others, appearing as a macro eNB to all connected HeNBs and/or eNBs. This sort of functionality is conceptually very similar to the X2 proxy functionality introduced in DeNBs (Donor ENBs) to support relaying functionality in LTE Rel-10. See, for instance, the technical specification 3GPP TS 36.300.
This approach, however, has several problems. First of all, the introduction of the X2-GW requires modifications to the 3GPP X2AP protocol (i.e. 3GPP X2 Application Protocol), with the risk that the procedures implemented through an X2-AP might diverge from the same procedures implemented on a direct connection. The ultimate risk is an additional cost for macro eNB vendors, who would have to test their eNBs for X2AP twice, first towards other (H)eNBs, and then in the presence of an X2-GW. Second, the dependence on 3GPP releases implies that this concept cannot be reused with older, i.e. earlier, releases, requiring an update of eNBs and HeNBs across the board. Third, deploying an X2-GW requires deploying a HeNB-GW also in cases that did not require it, increasing cost for operators. Fourth, by being co-located with the HeNB-GW, the X2-GW forces X2 signaling to follow strictly the same path as S1 signaling deep into the core network, thereby increasing traffic on the backhaul links and reducing the performance of X2 handovers, i.e. handovers over the X2 interface, notably much faster, over S1 handovers, i.e. handovers over the S1 interface.
Another possibility is to perform concentration at SCTP level, i.e. at transport layer, instead of at X2AP level, i.e. at application layer, introducing an SCTP concentrator. See, for instance, R3-120321, “SCTP Concentrator: A Simple Solution to a Debated Problem”, Ericsson, Alcatel-Lucent. The SCTP concentrator leverages the multi-homing and multi-stream capabilities of the SCTP protocol. See, for instance, Internet Engineering Task Force Request For Comments (IETF RFC) 4960: “Stream Control Transmission Protocol” (September 2007).
In this case, all the disadvantages of the X2-GW may disappear, because an SCTP concentrator would be transparent to X2AP and independent of 3GPP releases, and could be deployed separately from the HeNB-GW if present, hence giving the operator the maximum flexibility. With the deployment of an SCTP concentrator between the eNB and the HeNB, the X2 interface is still between the two nodes, but the SCTP concentrator may terminate the underlying SCTP connection. In order for the X2 interface to be set up, however, two separate SCTP associations generally need to be set up, and there is currently no way to set up X2 autonomously in such a scenario. Manual configuration or pre-configuration via Operations & Maintenance (OAM) would be impractical.